The use of gaseous fuel in internal combustion engines helps reduce engine out emissions due to more complete combustion; however, gaseous fuel displaces fresh air and thus limits peak torque as compared with liquid fuel. One approach is to use turbocharging to boost engine fresh intake air in order to compensate for the air displaced by the gaseous fuel. While turbocharging can address the fresh air displacement, it may lead to delayed engine response due to turbocharger lag.
The inventors herein have recognized that, contrary to conventional thinking, the displacement of fresh air by gaseous injection can be used to advantage in certain circumstances, particularly in systems having the ability to deliver fuel from at least two injectors to a cylinder, one of them being gaseous fuel and another being liquid fuel. Specifically, the inventors herein have recognized that it may be possible to utilize changes in the displacement of fresh air to reduce the effects of turbocharger delay.
In one example approach, a method of controlling an engine having a cylinder is provided. The method comprises injecting a gaseous fuel to a cylinder of the engine; directly injecting liquid fuel to the cylinder of the engine; boosting intake air delivered to the cylinder via a turbocharger coupled to the engine; and when increasing turbocharger boosting from a reduced boost level, decreasing the gaseous fuel injection while increasing liquid fuel injection to increase cylinder fresh air.
By decreasing the gaseous fuel injection while increasing liquid fuel injection when increasing turbocharger boosting, it is possible to take advantage of the displacement of fresh by gaseous fuel. Namely, by decreasing the gaseous fuel injection, the cylinder can be filled with a correspondingly increased fresh air amount, and then the extra air can be fueled via the liquid injector, even before the turbocharger has spun up, since the liquid fuel displaces substantially less fresh air than the gaseous fuel. In the example of compressed natural gas, which displaces around 13% of fresh air, if natural gas injection is completely stopped, this enables up to 13% more fresh air to the cylinder, and thus up to approximately 13% more torque to respond to a driver tip-in, for example, even before the turbocharger has spun up. Such operation can thus reduce the effects of turbocharger delay.
In addition, the ability to adjust cylinder torque by manipulating the displacement of fresh air by injected gaseous fuel can be expanded to various other torque control issues, not just turbocharger lag compensation. Thus, in another example, a method of controlling torque of an engine having a cylinder comprises: adjusting a cylinder air amount while maintaining a stoichiometric air-fuel ratio, the adjusting including: adjusting a displacement of fresh air by adjusting a gaseous fuel injection to the cylinder; and adjusting liquid fuel injection to the cylinder based on the adjusted fresh air in the cylinder.
In this way, it is possible to take advantage of the displacement of fresh air by a gaseous fuel, while also maintaining stoichiometry during the torque adjustment operation. Such operation can enable adjusting of the amount of fresh air in the cylinder by manipulation of the gaseous injection in coordination with liquid fuel injection to maintain a desired air-fuel ratio.
Note that maintenance of stoichiometry may include oscillation of exhaust air-fuel ratio round the stoichiometric ratio, e.g., due to feedback from exhaust air-fuel ratio sensors. In an alternative example, the method may maintain another desired air-fuel ratio, such as a desired lean air-fuel ratio.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.